How do I split Fusion drive on Mojave / APFS?

Hi Dan I am running Catalina and when trying to split the fusion contact net I get an error unable to mount the drive due to running process launchd Any idea what I need to do?

You're facing the cobblers dilemma! The cobbler can't fix your shoe while you are wearing them! Here the OS is using the drive you are trying to alter.

You need to boot up on an external bootable drive so nothing is accessing the Fusion Drive set drives.

I think you'er mixing things which don't really work.

● A Fusion Drive is when a small PCIe/NVMe SSD is setup as a cache drive to a slower SATA drive. Trying to create a dual SSD Fusion Drive won't be effective, the I/O throughput will be somewhere halfway between the two I/O throughputs.

● The speed of the PCIe/NVMe drive is much faster than the SATA III (6.0 Gb/s) HDD or SSD your system also supports. Setting up a dual independent drive config is by far the best setup internally. Here we get at least a 512 GB blade SSD and then either use the HDD or if you need fast data throughput (large images or vids) then using a SATA SSD.

● We also have one more drive interface: The external Thunderbolt 2 port which is even faster than either internal interfaces! As an example I have a 2013 MacPro (Trash Can) which has a 2 TB PCIe blade SSD and a set of external TB2 drives my active data drive is a RAIDed SSD and my second is a RAIDed HDD. I have only my OS, Apps & apps scratch space on the internal blade SSD and my current project is on the external SSD. The majority of my internal SSD is left empty so the OS and apps can leverage it for virtual RAM & caching.

And yes you will need to break the Fusion Drive set.

You may want to review RAID architectures as that's the way I would go if you really need the performance! Otherwise just use a dual drive config with a larger blade SSD and your current HDD or if really needed a SATA SSD.

No one has found dual SSD Fusion Drives workable! Some have tried setting up a JBOD set but that doesn't really work either Non-RAID drive architectures as the I/O speed difference between the two internal I/O interfaces is not the same causing sporadic I/O.

Forget the idea of fusing multiple SSDs together. As @Dan mentioned, it is not worth it.

Regarding your other questions:

You do not need to split the fusion drive via terminal beforehand.

Nor do you need to format the SSD you are installing, beforehand.

– Physically install the new SSD in place of the 1TB spinning disk

– Upon boot, hold option + R to boot into internet recovery, or, boot to a Mac USB installer

– While in recovery mode, Open Disk Utility and format the new SSD you installed. Also format the internal Apple "blade" SSD.

– Then, when you boot via your USB installer, begin the Catalina install process, and select the newly installed SSD as your destination

– You will wind up with 2 drives. (1) Your newly installed SSD, where the Mac OS is installed. (2) The old blade SSD, empty, which you can use for whatever.

Further thoughts:

– It isn't worth the time, effort, performance "gain", or added complexity (albeit, it isn't much to begin with) to "partition" the SSD into multiple drives. (Ie. Installing the Mac OS on one partition, and user/data and apps on the second partition – or whatever flavor of slicing and dicing you're thinking of)

– Use the leftover blade SSD for fast scratch space (video editing programs, audio, etc.)

Thank you both the speedy replies. I've read a ton on subject, but really needed a bit more clarification before I cracked my machine open. Nice to have multiple options for reconfiguring.

To clarify, I'm swapping my HDD for SDD because the HDD is failing and I really needed to get more life out of this machine before I drop another couple grand on a newer machine (I typically buy refurbs). I'm hoping I'm able to do that with the replacement drive. While I have it open, would you recommend checking any other components for wear/issues before I close it back up?

For future reference when the time comes, would you both recommend getting an iMac with the largest single SSD I can afford (vs another fusion)? I assume single SSD machines are PCIe and not SATA?

I've seen problems when the drives haven't be broken apart if you try reusing them. If you take both out then you won't need to worry. Both the SSD and the HDD have pointers in their master block areas which is why you need to alter the setting.

Review this guide: The Ultimate Guide to Apple’s Proprietary SSDs. If you can get a 512 GB or larger blade SSD and then keep the blade as your boot drive. Forget Fusion Drives! At the time they came out the cost of SSD's was still quite high and was a way to gain the speed but not the cost. Today SSD's are cheaper! Sadly, the custom Apple SSD's are still high even the 3rd party SSD's from OWC and Transcend. Don't get suckered into getting an M.2 if you really want something to last.

Yes! Get a SSD only iMac for your next system. But, don't go crazy going with the biggest!

Dual drive configs are still better! The issue is the amount of read/writes a drive goes through. SSD's have a limit unlike HDD's. While HDD's do have a weakness being mechanical most deep storage is still off of HDD's!

Don't get me wrong here I love SSD's too! It gets down to which tool is best for the condition and the risks if you get into trouble. If you are a heavy creator you want the dual drive setup. If you're just a dabbler then a single SSD drive is all you'll need.

No need to jump to another OS, besides Arch Linux does not support APFS file system it only can work with HFS+

Again, you DO NOT need to do this. It is totally optional.